Method of manufacturing buried heterostructure semiconductor laser

ABSTRACT

According a method of manufacturing a buried heterostructure semiconductor laser, an active layer and a p-type cladding layer are sequentially deposited on an n-type group III-V semiconductor layer by metalorganic vapor phase epitaxy. A surface of the deposited layer is masked in a stripe shape, and the cladding layer, the active layer, and the semiconductor layer are selectively and partially etched to form a mesa structure. A p-type current blocking layer, an n-type current confining layer containing a group VI dopant having a concentration of not less than 5×10 18  atoms·cm -3 , a p-type cladding layer, and a p-type cap layer are sequentially deposited on an entire upper surface of the mesa structure by the metalorganic vapor phase epitaxy.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing a buriedheterostructure semiconductor laser and, more particularly, to a methodof manufacturing a buried heterostructure semiconductor laser usingmetalorganic vapor phase epitaxy.

When a buried heterostructure semiconductor laser is to be manufactured,the step of burying a mesa structure including an active region isrequired. When this step is performed by metalorganic vapor phaseepitaxy, since metalorganic vapor phase epitaxy is a mesa transportmechanism, abnormal growth occurs at both the ends of the mesastructure. For this reason, it is difficult to bury the mesa structureflat.

In a conventional technique, as shown in FIG. 7, a mesa structure havinga small height (h<1 μm) is used, and growth of a buried layer isrepeated twice to form a buried heterostructure laser element.Alternatively, as shown in FIG. 8, when a mesa structure having a largeheight is to be used, a selection mask 14 on the upper portion of themesa structure is formed as an overhang to suppress the growth of boththe ends of the mesa structure, and buried layers are grown to form thelaser structure. In FIGS. 7 and 8, reference numeral 11a denotes ann-type InP substrate; 11b, an Se-doped n-type InP buffer layer on thesubstrate 11a; 12, an undoped InGaAsP active layer; 13, a p-type InPcladding layer; and 14, an SiO₂ film for forming a selection mask. Inaddition, reference numeral 15 denotes a p-type InP current blockinglayer; 16, an n-type InP current confining layer; 17, a p-type InPover-cladding layer; and 18, a p-type InGaAsP cap layer.

According to the conventional technique using the mesa structure havinga small height shown in FIG. 7, however, a film thickness of 1.2 μm ormore is required to sufficiently block a current by a p-n reverse biasof buried layers consisting of the p-type InP current blocking layer 15and the n-type InP current confining layer 16. As a result, as shown inFIG. 7, the buried layers largely protrude (1.0 μm or more) at both theends of the mesa structure. When the buried layers protrude at both theends of the mesa structure, it is difficult that a buried layer is grownby the second growth of a buried layer to flatten the surface of anelement, and trouble may occur in the steps of isolating electrodes andelements.

In addition, according to the conventional technique using the selectionmask having an overhang, as shown in FIG. 8, the mesa structure must beformed by wet etching, and the controllability of a mesa shape is notgood. For this reason, uniformity and controllability of lasercharacteristics are degraded and the yield of lasers is decreasedaccordingly.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a method ofmanufacturing a high-performance buried heterostructure semiconductorlaser capable of obtaining a flat buried layer.

It is another object of the present invention to provide a method ofmanufacturing a buried heterostructure semiconductor laser in which ahigh-performance semiconductor laser can be manufactured by simplemanufacturing steps.

In order to achieve the above objects, according to the presentinvention, there is provided a method of manufacturing a buriedheterostructure semiconductor laser, comprising the steps ofsequentially depositing an active layer and a p-type cladding layer onan n-type group III-V semiconductor layer by metalorganic vapor phaseepitaxy, masking a surface of the deposited layer in a stripe shape andselectively and partially etching the cladding layer, the active layer,and the semiconductor layer to form a mesa structure, and sequentiallydepositing a p-type current blocking layer, an n-type current confininglayer containing a group VI dopant having a concentration of not lessthan 5×10¹⁸ atoms·cm⁻³, a p-type cladding layer, and a p-type cap layeron an entire upper surface of the mesa structure by metalorganic vaporphase epitaxy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are sectional views showing the steps in manufacturing aburied heterostructure semiconductor laser according to an embodiment ofthe present invention;

FIGS. 2A to 2C are sectional views showing the steps in manufacturing aburied heterostructure semiconductor laser according to anotherembodiment of the present invention;

FIGS. 3A to 3D are sectional views showing the steps in manufacturing aburied heterostructure semiconductor laser according to still anotherembodiment of the present invention;

FIGS. 4A to 4D are sectional views showing the steps in manufacturing aburied heterostructure semiconductor laser according to still anotherembodiment of the present invention;

FIGS. 5 and 6 are graphs each showing concentration dependency on thegrown film thickness of an Se-doped InP current confining layer; and

FIGS. 7 and 8 are sectional views for explaining element structures ofconventional buried heterostructure semiconductor lasers, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A to 1D show the steps in manufacturing a buried heterostructuresemiconductor laser according to an embodiment of the present invention.

As shown in FIG. 1A, an Se-doped n-type InP buffer layer 1b (filmthickness: d=0.1 μm), an undoped InGaAsP active layer 2 (d=0.1 μm), anda p-type InP cladding layer 3 (d=0.3 μm) are grown on a (100) planen-type InP substrate 1a by metalorganic vapor phase epitaxial growth(MOVPE).

An SiO₂ film 4 is deposited on the grown surface of the resultantstructure by sputtering.

As shown in FIG. 1B, an SiO₂ stripe mask 4A having a stripe width of 1.5μm is formed in the <011> direction by a photolithographic technique.The cladding layer 3, the active layer 2, the buffer layer 1b, and thesubstrate 1a are partially removed by etching using a chlorine-argonreactive ion etching (RIE) device to form a mesa structure having aheight of about 0.1 μm. At this time, since this mesa structure isformed by all dry processes, the shape of the mesa structure has highcontrollability.

As shown in FIG. 1C, the SiO₂ stripe mask 4A formed by the SiO₂ film 4constituting the upper layer of the mesa structure is directly used as aselective growth mask, and a Zn-doped p-type InP current blocking layer5 and an Se-doped n-type InP current confining layer 6 are sequentiallygrown by MOVPE to bury regions other than the mesa structure. The p-typeInP layer 5 and the n-type InP layer 6 serve as a current constraintlayer and an optical confinement layer. At this time, the concentrationof Se doped in the n-type InP layer 6 is set to be 5×10¹⁸ atoms·cm⁻³ ormore. In this manner, since the n-type InP buried layer 6 on the sidesurface of the mesa structure is grown while its (100) facet is exposed,and the growth of the (100) facet plane is suppressed. For this reason,since a (100) off-angle portion is grown at a high rate, the surface ofthe buried layer on the side surface of the mesa structure has theexposed (100) plane so as to be flattened. The SiO₂ film constitutingthe upper portion of the mesa structure is removed by HF.

Thereafter, as shown in FIG. 1D, a p-type over-cladding layer 7 (d=1.0μm) and a p-type InGaAsP cap layer 8 (d=0.5 μm) are sequentially grownon the entire surface of the substrate by MOVPE. Since the substantiallyflat crystal surface is obtained when the first growth of a buried layeris performed, an element structure having a flat surface can be obtainedafter the second growth of a buried layer is performed on the entiresurface of the substrate.

As described above, since high-concentration n-type InP containing agroup VI dopant is used, the flat buried layer can be obtained, and ahigh-performance semiconductor layer can be obtained.

The growth of InP is suppressed in the small region on the (100) planewhen the Se-doped n-type InP current confining layer 6 is formed,because Se eleminates the surface level of the (100) plane of the InP tostabilize the (100) plane. The surface smoothing effect is obtained byadsorbing group VI atoms (Se and S) in the surface of a group III-Vsemiconductor.

As a device used in MOVPE, a high-frequency heating low-pressurevertical metalorganic vapor phase epitaxial growth device was used. Agrowth temperature was set to be 620° C., and a growth pressure was setto be 50 Torr. The following materials were used as source gases. Forexample, hydrides of P and As such as phosphine PH₃ and arsine AsH₃ wereused as group V sources, and organic metals of In and Ga such as TMInand TEGa were used as group III sources. DEZn and H₂ Se were used as thep-type dopant and the n-type dopant, respectively. In addition, a growthrate was set to be 330 Å/min (2.0 μm/h) during burying of InP, and aV/III ratio (PH₃ /TMIn) was set to be 120.

The concentration dependency on the grown film thickness of the growthof the Se-doped n-type InP layer 6 is given as a characteristic curveshown in FIG. 5. In FIG. 5, reference symbol w represents the width ofthe top of the mesa structure; d, the thickness of a portion of thecurrent confining layer 6 adjacent to the mesa structure; and d₀, thethickness of a portion of the current confining layer 6 apart from themesa structure. According to this characteristic curve, the following isfound. That is, when the concentration of doped Se is set to be 5×10¹⁸atoms·cm⁻³ or more, d/d₀ has a sufficiently small value, and the growthof the (100) facet plane is sufficiently suppressed.

FIG. 6 shows concentration dependency on the grown film thickness of theSe-doped n-type InP layer 6 formed on the p-type InP layer 5 when thewidth w of the top of the mesa structure is varied, and FIG. 6 shows arelationship between the concentration dependency of the Se-doped n-typeInP layer 6 and the concentration dependency of an Si-doped n-type InPlayer which is obtained to be compared with that of the Se-doped n-typeInP layer 6. As is apparent from FIG. 6, it is understood that a growthsuppressing effect is larger when Se is doped than when Si is doped. Inaddition, as the width of the top of the mesa structure is varied, i.e.,as the width is decreased, the growth suppressing effect becomes large.

FIGS. 2A to 2C show the steps in manufacturing a semiconductor laseraccording to another embodiment of the present invention. The samereference numerals as in FIGS. 1A to 1D denote the same parts in FIGS.2A to 2C.

As shown in FIG. 2A, an Se-doped n-type InP buffer layer 1b (d=0.1 μm),an undoped InGaAsP active layer 2 (d=0.1 μm), a p-type InP claddinglayer 3 (d=1.2 μm), and a p-type InGaAsP cap layer 8 (d=0.5 μm) aregrown on a (100) plane n-type InP substrate 1a by MOVPE. An SiO₂ film 4is deposited on the grown surface by sputtering.

As shown in FIG. 2B, an SiO₂ stripe mask 4A having a stripe width of 1.5μm is formed in the <011> direction by a photolithographic technique. Achlorine-argon reactive ion etching (RIE) device is used to form a mesastructure having a height of about 2.0 μm.

The SiO₂ stripe mask 4A formed by the SiO₂ film 4 constituting the upperlayer of the mesa structure is directly used as a selective growth mask,and a Zn-doped p-type InP current blocking layer 5 and an Se-dopedn-type InP current confining layer 6 are sequentially grown by MOVPE tobury regions other than the mesa structure, as shown in FIG. 2C. Thep-type InP layer 5 and the n-type InP layer 6 serve as a currentconstraint layer and an optical confinement layer. At this time, theconcentration of Se doped in the n-type InP layer 6 is set to be 5×10¹⁸atoms·cm⁻³ or more. In this manner, since the n-type InP buried layer 6on the side surface of the mesa structure is grown while its (100) facetis exposed, the growth of the (100) facet plane is suppressed, and a(100) off-angle portion is grown at a high rate. For this reason, thesurface of the buried layer has the exposed (100) plane so as to beflattened. The SiO₂ film 4A constituting the upper portion of the mesastructure is removed by HF.

Since the element manufactured as described above uses thecharacteristic features of growth of a buried layer, the mesa structureis not limited, and a laser element having a flat surface can bemanufactured.

FIGS. 3A to 3D show a method in manufacturing a semiconductor laseraccording to still another embodiment of the present invention. The samereference numerals as in FIGS. 1A to 1D denote the same parts in FIGS.3A to 3D. This embodiment has the following principal characteristicfeature. A group VI dopant such as Se is doped in an n-type buried layerin the step of burying a mesa structure using metalorganic vapor phaseepitaxy, and a growth suppressing mechanism of a (100) small regionwhich is a characteristic feature of the group VI dopant heavily dopedn-type group III-V compound semiconductor is used, thereby burying themesa structure without a selection mask.

As shown in FIG. 3A, an Se-doped n-type InP buffer layer 1b (filmthickness: d=0.1 μm), an undoped InGaAsP active layer 2 (d=0.1 μm), anda p-type InP cladding layer 3 (d=0.3 μm), are grown on a (100) planen-type InP substrate la by metalorganic vapor phase epitaxy (MOVPE). AnSiO₂ film 4 is deposited on the grown surface by sputtering.

As shown in FIG. 3B, an SiO₂ stripe mask 4A having a stripe width of 1.5μm is formed in the <011> direction by a photolithographic technique. Achlorine-argon reactive ion etching (RIE) device is used to form a mesastructure having a height of about 1.0 μm. The SiO₂ film 4A constitutingthe upper layer of the mesa structure is removed by HF.

As shown in FIG. 3C, a Zn-doped p-type InP current blocking layer 5 andan Se-doped n-type InP current confining layer 6 are sequentially grownby MOVPE. The p-type InP layer 5 and the n-type InP layer 6 serve as acurrent constraint layer and an optical confinement layer. At this time,when the concentration of Se doped in the n-type InP layer 6 is set tobe 5×10¹⁸ atoms·cm⁻³ or more, the growth of the n-type InP buried layer6 on the mesa structure is suppressed. For this reason, the upperportion of the mesa structure has a layered structure in which only thep-type InP layer 5 is grown. As is apparent from FIG. 6, when theconcentration of Se doping is set to be 8×10¹⁸ atoms·cm⁻³ or more, thegrowth of the buried layer 6 on the mesa structure is entirelysuppressed.

Sequentially, as shown in FIG. 3D, a p-type InP over-cladding layer 7(d=1.0 μm) and a p-type InGaAsP cap layer 8 (d=0.5 μm) are sequentiallygrown on the entire surface of the substrate by MOVPE. The p-type Inplayer 7 and the p-type InGaAsP layer 8 are also grown on the mesastructure and constitute the element structure.

In the element manufactured as described above, since thehigh-concentration n-type InP layer 6 using an Se dopant is used, acondition under which the n-type InP layer 6 is not formed on the mesastructure can be obtained. For this reason, a buried heterostructurelaser element can be manufactured by one growth of a buried layerperformed by MOVPE without a selection mask.

FIGS. 4A to 4D show a method in manufacturing a semiconductor laseraccording to still another embodiment of the present invention. The samereference numerals as in FIGS. 1A to 1D denote the same parts in FIGS.4A to 4D.

As shown in FIG. 4A, an Se-doped n-type InP buffer layer 1b (d=0.1 μm)and an undoped InGaAsP active layer 2 (d=0.1 μm) are grown on a (100)plane n-type InP substrate 1a by MOVPE.

As in the embodiment shown in FIGS. 3A to 3D, a mesa structure having nomask in the <011> direction is formed (FIG. 4B), and a Zn-doped p-typeInP current blocking layer 5 and an Se-doped n-type InP currentconfining layer 6 are grown using MOVPE (FIG. 4C). Subsequently, ap-type InP over-cladding layer 7 (d=1.0 μm) and a p-type InGaAsP caplayer 8 (d=0.5 μm) are grown on the entire surface of the substrate toform an element structure (FIG. 4D).

In the element manufactured as described above, since a selection maskis not used, a buried heterostructure laser element can be manufacturedby one growth of a buried layer as in the embodiment shown in FIGS. 3Ato 3D.

In the embodiment shown in FIGS. 4A to 4D, after the undoped InGaAsactive layer 2 is grown (FIG. 4A), an optical waveguide layer is grown.Thereafter, a diffraction grating is formed in the optical waveguidelayer, and a mesa structure is formed as in the step in FIG. 4B. Adistributed feedback laser element is manufactured in the same steps asthose in FIGS. 4C and 4D. In this case, a regrowth of the p-type InPcladding layer performed after the diffraction grating is formed can beadvantageously omitted.

As described above, according to the embodiments shown in FIGS. 3A to 3Dand FIGS. 4A to 4D, when a growth suppressing effect in a small regionof the (100) plane of a heavily doped n-type III-V group semiconductorusing a group VI dopant such as Se is used, a mesa structure having nomask for selective growth can be buried by one growth using metalorganicvapor phase epitaxy. For this reason, the steps in manufacturing aburied heterostructure laser can be advantageously simplified.

In the above embodiments, although a chlorine-argon dry etching is usedas a method of forming a mesa structure, the mesa structure may beformed by another method.

In the above embodiments, although Se is used as a group VI dopant,another group VI dopant such as S may be used because the surfacesmoothing effect can be obtained by using not only Se but other group VIelements.

In addition, in the above embodiments, the InP/InGaAsP semiconductorshave been described above. However, any other group III-V semiconductorssuch as GaAs/AlGaAs semiconductors having the surface smoothing effect(the effect is confirmed in the GaAs/AlGaAs semiconductors) can beapplied to the present invention.

What is claimed is:
 1. A method of manufacturing a buriedheterostructure semiconductor laser, comprising the stepsof:sequentially depositing an active layer and a p-type cladding layeron an n-type group III-V semiconductor layer by metalorganic vapor phaseepitaxy; masking a surface of said deposited layer in a stripe shape andselectively and partially dry etching said cladding layer, said activelayer, and part of said semiconductor layer to form a mesa structure;and sequentially depositing a p-type current blocking layer, an n-typecurrent confining layer containing a group VI dopant having aconcentration of not less than 5×10¹⁸ atoms·cm⁻³, a p-type claddinglayer, and a p-type cap layer on an entire upper surface of said mesastructure by the metalorganic vapor phase epitaxy.
 2. A method accordingto claim 1, wherein said n-type semiconductor layer consists of ann-type semiconductor substrate and an n-type buffer layer formed on saidn-type semiconductor substrate.
 3. A method according to claim 1,wherein the step of selectively and partially etching said claddinglayer, said active layer, and said semiconductor layer to form a mesastructure is performed by a reactive ion etching method.
 4. A methodaccording to claim 1, wherein said n-type group III-V semiconductorlayer consists of InP.
 5. A method according to claim 1, wherein saidcurrent confining layer uses an Se dopant having a concentration of notless than 5×10¹⁸ atoms·cm⁻³.
 6. A method according to claim 1, whereinsaid mask is removed after said current confining layer is formed.
 7. Amethod according to claim 1, wherein said mask is removed after saidmesa structure is formed.
 8. A method according to claim 7, wherein saidcurrent confining layer uses an Se dopant having a concentration of notless than 8×10¹⁸ atoms·cm⁻³.
 9. A method of manufacturing a buriedheterostructure semiconductor laser, comprising the stepsof:sequentially depositing an active layer, a p-type cladding layer, anda p-type cap layer on an n-type group III-V semiconductor layer by themetalorganic vapor phase epitaxy; masking a surface of said depositedlayer in a stripe shape and selectively and partially dry etching saidcap layer, said cladding layer, said active layer, and saidsemiconductor substrate to form a mesa structure; and sequentiallydepositing a p-type current blocking layer, an n-type current confininglayer containing a group VI dopant having a concentration of not lessthan 5×10¹⁸ atoms·cm⁻³ by the metalorganic vapor phase epitaxy to formcurrent constraint and optical confinement layers.
 10. A methodaccording to claim 9, wherein said n-type semiconductor layer consistsof an n-type semiconductor substrate and an n-type buffer layer formedon said n-type semiconductor substrate.
 11. A method according to claim9, wherein the step of selectively and partially etching said claddinglayer, said active layer, and said semiconductor layer to form a mesastructure is performed by a reactive ion etching method.
 12. A methodaccording to claim 9, wherein said n-type group III-V semiconductorlayer consists of InP.
 13. A method according to claim 9, wherein saidcurrent confining layer uses an Se dopant having a concentration of notless than 5×10¹⁸ atoms·cm⁻³.
 14. A method according to claim 9, whereinsaid mask is removed after said current confining layer is formed.
 15. Amethod of manufacturing a buried heterostructure semiconductor laser,comprising the steps of:depositing an active layer on an n-type groupIII-V semiconductor layer by metalorganic vapor phase epitaxy; masking asurface of said active layer in a stripe shape and selectively partiallydry etching said active layer and said semiconductor substrate to form amesa structure; and removing said mask on the upper layer of said mesastructure, and depositing a p-type current blocking layer, an n-typecurrent confining layer containing a group VI dopant having aconcentration of not less than 5×10¹⁸ atoms·cm⁻³, a p-type claddinglayer, and a p-type cap layer on an entire surface of the resultantstructure by the metalorganic vapor phase epitaxy.
 16. A methodaccording to claim 15, wherein said n-type semiconductor layer consistsof an n-type semiconductor substrate and an n-type buffer layer formedon said n-type semiconductor substrate.
 17. A method according to claim15, wherein the step of selectively and partially etching said activelayer and said semiconductor layer to form a mesa structure is performedby a reactive ion etching method.
 18. A method according to claim 15,wherein said n-type group III-V semiconductor layer consists of InP. 19.A method according to claim 15, wherein said current confining layeruses an Se dopant having a concentration of not less than 8×10¹⁸atoms·cm⁻³.
 20. A method of manufacturing a buried heterostructuresemiconductor laser, comprising the steps of:depositing an active layeron an n-type group III-V semiconductor layer by metalorganic vapor phaseepitaxy; growing said active layer, growing an optical waveguide layer,and forming a diffraction grating in said optical waveguide layer;masking the surface of said optical waveguide layer in a stripe shapeafter the diffraction grating is formed, and selectively and partiallydry etching said optical waveguide layer, said active layer, and saidsemiconductor substrate to form a mesa structure; and removing said maskon the upper layer of said mesa structure, and depositing a p-typecurrent blocking layer, an n-type current confining layer containing agroup VI dopant having a concentration of not less than 5×10¹⁸atoms·cm⁻³, and a p-type cladding layer, and a p-type cap layer on anentire surface of the resultant structure by the metalorganic vaporphase epitaxy.
 21. A method according to claim 20, wherein said n-typegroup III-V semiconductor layer consists of InP.
 22. A method accordingto claim 20, wherein said current confining layer uses an Se dopanthaving a concentration of not less than 8×10¹⁸ atoms·cm⁻³.
 23. A methodof manufacturing a buried heterostructure semiconductor laser,comprising the steps of:sequentially depositing an active layer and ap-type cladding layer on an n-type group III-V semiconductor layer bymetalorganic vapor phase epitaxy; masking a surface of said depositedlayer in a stripe shape and selectively and partially dry etching saidcladding layer, said active layer, and said semiconductor substrate toform a mesa structure; and sequentially depositing a doped p-typecurrent blocking layer, a n-type current confining layer containing agroup VI dopant, a p-type cladding layer, and a p-type cap layer on anentire upper surface of said mesa structure by the metalorganic vaporphase epitaxy.